The Paleoclimatic Significances Of Shell Morphology Of Fossil Cathaica Orithyia From Chinese Loess Plateau

Paleoclimate reconstructions have been and continue to be subjects of intense scientific interest.Understanding the evolution patterns and the climate influences of the Asian Summer Monsoon(ASM)depends on multiple geological records from Chinese monsoon region.The Chinese speleothem ?18O records indicate that the ASM intensity is mainly driven by the variation of Northern Hemisphere summer insolation which is dominated by precession cycles(23-ka).However,the loess-based paleoclimate proxies from the Chinese Loess Plateau(CLP)in northern China reveal that the variations of ASM intensity is dominated by the 100-ka cycle since the mid-Pleistocene transition.According to the speleothem ?18O-based viewpoint,the ASM intensity variations are driven by the low-latitude hydrology process,while the loess records-based studies suppose that the ASM intensity variations is driven by the changing of ice volume in high-latitude area.The discrepancies in patterns and forcing mechanisms of ASM showed by different paleoclimate proxies may result from the limitation of single paleoclimate proxy on recording climate information from all aspects.In order to fully understand the ASM intensity variations and the related climatic changes,more independent paleoclimate indicators with totally different mechanisms are needed,which will be helpful for understanding the evolution patterns and the forcing mechanisms of the ASM.Land snail is the most abundant biological remains in loess deposit from the CLP.It has a great potentiality on paleoclimate reconstruction in northern China.Using the species assemblage and the isotope composition methods,land snail fossils have contributed much to reconstruct paleoclimate in the ASM region.However there is very few work which reconstructs paleoclimate in respect of the biological mechanisms or the interaction between land snails and environment.Here,I studied the shell morphology variations of fossil land snail Cathaica orithyia(Martens,1879)from the CLP,discussing the response of shell morphology to environmental and climate changes.The shell shape analyses of fossil C.orithyia over the past 470 ka from Xifeng in the central CLP shows that shell shape variations mainly focus on two aspects—the shift of the upper part of aperture(UPA),and the spire index of shell(shell height : shell width).Therein,the shift of UPA includes the longitudinal shift and the lateral shift.Both are able to regulate the antidrought ability of shell of C.orithyia.The longitudinal shift of UPA which reflects the aperture deflection of C.orithyia shows a significantly positive correlation with the Chinese speleothem?18O on precession time scale,indicating an increasing anti-drought ability of C.orithyia with the increasing of speleothem ?18O,supporting the viewpoint that the Chinese speleothem ?18O is a precipitation indicator of northern China.The lateral shift of UPA shows a good correlation with the stimulated precipitation based on the loess microcodium Sr/Ca ratio from Xifeng which is dominated by obliquity cycles(41-ka),indicating a strong forward extent of UPA during drought period,representing a better anti-drought function of C.orithyia shell.In summary,the shape variations of aperture indicate that precipitation in northern China follows the precession cycle and the obliquity cycle simultaneously.Except providing the precipitation signals,the land snail shell shape also shows the temperature signal during the glacial-interglacial cycles.However,because the shape variation responding to this temperature signal is unclear,the mechanism between shell shape and temperature cannot be discussed.The shell morphology analyses of C.orithyia from 80 ka B.P.to 20 ka B.P.from Mangshan,northern China indicate that shell shape of C.orithyia has a good correlation with the climate changes on sub-orbital time scale.The shape variations of C.orithyia from Mangshan also mainly focus on the shift of UPA and the spire index.The aperture shape during the interim between Heinrich 6 and Heinrich 5 shows a growing anti-drought ability,suggesting a long lasting and strong drought event.In the last stage of this drought event,the C.orithyia disappeared from the loess deposits,indicating a regional extinction of C.orithyia in Mangshan.The variation of shell spire index of C.orithyia responds well to the Heinrich Events that represent the millennial cooling events.The shell became more globose during the Heinrich Event,which may result from that the cooling event constrains the local plant's growth,and forces C.orithyia to choose a more globose shell shape to improve its mobility in foraging.The present work indicates that land snail shell shape is a complex climate proxy that records several environment and climate information on different time scale.By using the geometric morphological methods,these environment and climate information can be separated,which is helpful for understanding the evolution patterns and the forcing mechanisms of the ASM,and the interaction between different climate factors.